The present invention relates to an optical coupler with a new geometry and a new principle of action. The optical coupler provides spectrally selective coupling of light to or from a waveguide, such as an optical fibre.
Spectrally selective optical couplers, also known as channel drop filters, are utilised for extraction of a single wavelength channel from a broadband optical signal, or for insertion of a single wavelength channel into a broadband optical signal. Typically, spectrally selective couplers are used in wavelength division multiplexed optical communications systems for adding and dropping a single wavelength channel.
Channel drop filters have previously been implemented as dual-waveguide couplers. The article xe2x80x9cNarrow-Band Optical Channel-Dropping Filterxe2x80x9d, Journal of Lightwave Technology, vol. 10, no. 1, January 1992 (Haus et al.) describes an optical channel-dropping filter comprising a first and a second waveguide, the first of which contains a xcex/4 shifted distributed feedback (DFB) resonator. Light propagating in the second waveguide is coupled to the first waveguide by evanescent coupling between the two waveguides. Only one wavelength of light is resonant in the first waveguide, and consequently only that wavelength of light is efficiently coupled to the first waveguide. By making the xcex/4 shifted DFB resonator asymmetric (i.e. the grating is longer on one side of the xcex/4 shift), light can be coupled out of the DFB resonator.
However, prior art channel drop filters have some significant drawbacks and limitations. The filters are difficult to manufacture, due to the fact that very precise placement of the waveguides is required, in order to obtain a reliable evanescent coupling. Furthermore, the prior art filters are difficult to control. The coupling strength and the coupled wavelength is, to a large extent, fixed once the device is assembled. Also, each filter needs to be of a certain size in order to achieve the necessary feedback. In particular, when a number of channels are to be dropped separately (e.g. when constructing a demultiplexer), the device needs to be quite large. Yet another problem with the prior art filters is that they are difficult to implement in a fibre configuration, since the evanescent coupling between the waveguides needs to be very accurate. Any perturbation of either of the waveguides can cause large uncontrolled changes in performance.
The present invention provides an optical coupler with a new geometry and a new principle of action, which eliminates, or at least alleviates, the aforementioned problems in the prior art.
An optical coupler according to the present invention comprises an optical waveguide, preferably an optical fibre, in which there is provided a deflector for deflecting at least some of the light, propagating in said waveguide, into an external resonator. By external resonator, it is meant that the resonator is defined by mirrors that are arranged outside the optical waveguide. The external resonator is arranged to be resonant to a predetermined wavelength. The resonance of the predetermined wavelength in the external resonator causes an energy build-up of said wavelength in said resonator. In effect, the coupling of the resonant wavelength is rendered much stronger than what is obtained by the deflector alone. Owing to this, the deflector can be made very weak, deflecting only a small fraction of the light in the waveguide. The resonant wavelength can, in a very advantageous way, be coupled out of the external resonator. Light of a wavelength that is not resonant in the external resonator is essentially unperturbed by the coupling, since only a very small fraction of the light in the waveguide is deflected by the deflector.
A general insight, forming a basis for the present invention, is that coupling of light is much more efficient and spectrally selective when performed resonantly. This fact is utilised in the present invention by arranging an external resonator outside an optical waveguide. In the waveguide, there is arranged a deflector, deflecting light into the external resonator. Consequently, light being resonant in the external resonator is more strongly coupled (by the deflector) between the waveguide and the resonator.
Hence, a very weak coupling factor can be used if the coupling is made in connection with a resonance to the wavelength at issue for coupling. Wavelengths not exhibiting resonance in the coupling region are essentially unperturbed, due to the very weak coupling factor for non-resonant wavelengths.
In one aspect, the present invention provides a spectrally selective optical coupler, comprising a waveguide with a deflector. The deflector is operative to deflect light from the light guiding structure of the waveguide into an external resonator, which external resonator is defined by two mirrors provided on opposite sides and outside of said light guiding structure. Light can be coupled out of the external resonator by either of the mirrors having a slightly reduced reflectivity, or by either of the mirrors being provided with an aperture of strongly reduced reflectivity (as compared to the reflectivity of the same mirror off said aperture).
In a preferred embodiment, the deflector comprises a periodic refractive index modulation in the light guiding structure of the waveguide. In the most preferred embodiment, this periodic refractive index modulation is a tilted optical Bragg grating. The tilted optical Bragg grating forms, effectively, a deflector comprising a cascaded series of very weak reflectors, each of which couples light into a single resonant mode in the external resonator.
In another aspect, the present invention provides a spectrally selective optical coupler that is tuneable, thus allowing coupling of different wavelengths at different instants by tuning said coupler. This is obtained by at least one of the mirrors defining the external resonator being adjustable, as to distance between the mirrors or to resonator angle with respect to the light guiding structure of the waveguide. By tilting the external resonator with respect to the light guiding structure, the inventive optical coupler can also be made to pass all wavelengths.
In yet another aspect, the present invention provides optical couplers with external resonators, in which the propagation direction of light is essentially perpendicular with respect to the light guiding structure of the waveguide. Thus, the spectrally selective optical couplers according to the present invention can easily be cascaded, in order to achieve coupling of different wavelengths at different positions. This is obtained by arranging a plurality of optical couplers in series, each coupler of said plurality of optical couplers being resonator to a different wavelength.
It will be appreciated by those skilled in the art that the features of the present invention governing the coupling of light out of an optical waveguide are equally applicable to the coupling of light into an optical waveguide, since the latter is merely the time reversal of the former.
According to the invention, the deflector that is used to deflect light into the external waveguide can be wavelength discriminating. However, the spectral selectivity of the deflector need not be very high, since spectral selectivity is mainly achieved by means of the external resonator. Nevertheless, in some cases, it might be desirable to have a degree of spectral selectivity in the deflector. This is easily achieved by using a deflector comprising a tilted or a transversally asymmetric Bragg grating, which is adapted for deflection of a predetermined wavelength in a predetermined direction. Generally, the deflected wavelength and its deflection angle are determined by the period of the Bragg grating.
In a transversally asymmetric Bragg grating, the amplitude (modulation depth) is lower at one edge of the grating (radially) than at the opposite edge. This means that when light is reflected against the grating it will have a direction of propagation which is somewhat different from the direction of incidence. If the transversal modulation depth variation is sufficiently large it will be possible to couple light to and from the waveguide with the aid of the transversally asymmetrical phase grating.
In other embodiments, the deflector comprises a surface roughness of the light guiding structure. For example, the surface roughness can be periodic grooves or other light-deflecting means. Also, the mirrors defining the external resonator can be provided with a surface roughness of this kind, to couple out light from the resonator.
One advantage of the optical coupler according to the present invention is that the coupler can be made very compact. The external resonator can advantageously be arranged in such a way that the light in the resonator propagates in a direction perpendicular to the light in the waveguide (sometimes in this application, this is referred to as the external resonator being perpendicular to the waveguide). Thus, light can easily be coupled to or from the waveguide perpendicularly. It will be appreciated that perpendicular coupling of light to or from a waveguide is the most compact way of coupling.
Another advantage of optical couplers according to the present invention is that a plurality of couplers can be arranged in series in a simple manner, thereby facilitating coupling of different wavelengths at different positions along the waveguide.
Yet another advantage of the present invention is its sturdiness. The deflector is incorporated into the waveguide and cannot be tampered with. In the case of the waveguide being an optical fibre, the mirrors defining the external resonator can conveniently be deposited onto the outer surface of the fibre cladding. However, and as known in the art, the curvature of the mirrors must match the waist of the resonator mode in the fibre core. In this case, tuning of the resonator wavelength is obtained by pressing the mirrors, i.e. the cladding. Alternatively, one or both of the mirrors can be separate from the cladding, in which case tuneability is even more easily achieved.
Furthermore, the geometry of the optical coupler according to the invention is very favourable for fibre based applications. It is envisioned that the two mirrors defining the external resonator actually are comprised of a single circumferential reflector, extending fully or partly around the fibre cladding. In such situation, a first part of said reflector constitutes the first mirror, and a second part of said reflector constitutes the second mirror. The requirement being that said two parts still defines a resonator. Typically, the first part and the second part of the reflector (i.e. the first and the second mirror) are opposed to each other, thereby being essentially parallel.
Since the inventive coupler can be made very small, temperature control of the components is more easily achieved. A small size also alleviates the relative precision requirements on the device.